1. Field of the Invention
The present invention relates to stilbene derivatives, light-emitting elements which use stilbene derivatives, display devices having the light-emitting elements, and electronic devices having the display devices. In more detail, the present invention relates to novel stilbene derivatives which can provide excellent color purity of blue; and light-emitting elements, display devices, and electronic devices which use the stilbene derivatives.
2. Description of the Related Art
Light-emitting elements have features such as low-voltage drive, high efficiency, and high luminance and are expected to be applied to flat panel displays of the next generation. It is considered that a light-emitting device in which light-emitting elements are used for a pixel portion is superior to a conventional liquid crystal display device in viewing angle and visibility.
A light-emitting element is formed by interposing a layer including a luminescent substance between a pair of electrodes (an anode and a cathode). An emission mechanism thereof is regarded as follows: when voltage is applied between the pair of electrodes, holes injected from the anode and electrons injected from the cathode recombine in a light-emitting layer including a luminescent substance, whereby a molecular exciton is formed, and energy is released when the molecular exciton relaxes to a ground state. Thus, light is emitted. A luminescent substance can exist in singlet excited state and a triplet excited state. Light emission from a singlet excited state is referred to as fluorescence and light emission from a triplet excited state is referred to as phosphorescence.
An emission wavelength of a light-emitting element is determined by an energy difference between a ground state and an excited state, that is, a band gap, of a light-emitting molecule included in the light-emitting element. Therefore, various emission colors can be obtained by devising a structure of the light-emitting molecule. By using the light-emitting elements capable of emitting red light, blue light, or green light, which are the three primary colors of light, a full-color light-emitting device can be manufactured. Accordingly, highly reliable light-emitting elements which are excellent in color purity and emit red light, blue light, or green light are being investigated.
As a result of recent development of materials, red emissive and green emissive light-emitting element with high reliability and excellent color purity have been realized. However, a blue emissive light emitting element having sufficient reliability and color purity has not yet been developed in spite of intensive research thereon (for example, see Reference 1: Japanese Published Patent Application No. 2004-75580).